The Marking of Gemstones
The previous inscription of a uniquely-defined identifying mark, or indicium, on a gemstone that has been stolen, lost, or mixed in a lot greatly facilitates its identification in case of recovery and its subsequent return to the rightful owner. As a result, insurance companies strongly encourage the marking of high-valued precious gemstones since most of these articles are insured. Likewise, inscribing an indicium that simply indicates the mining site or the country of origin of gemstones such as diamonds would be an efficient way to prevent from the entry of the so-called “conflict diamonds” in the legitimate diamond industry.
The marking of articles of various natures for purposes such as their unambiguous identification, classification, tracking, or ease of recovery is firmly established. The marked indicia can take the form of human-readable codes such as logos, artistic images, hallmarks, or serial numbers made from a stream of alphanumeric characters. Machine-readable codes such as the common 1-D bar codes or 2-D arrays of dot-like marks designed in accordance with various types of symbologies can be inscribed as well. Several distinguishing features of gemstones make their marking notoriously challenging. For example, indicia must be engraved on the surface of very small articles that generally comprise a large number of even smaller facets oriented in various directions. In addition, only a limited portion of the outer surface of a gemstone is accessible to marking when the stone is mounted in a setting. Adding to these difficulties is the fact that gemstones like diamonds are made from a material of extreme hardness while being subject to fracture upon sudden mechanical stress or excessive local heating. More importantly, inscribing a permanent indicium on a cut and polished gemstone must not impair its appearance, quality, and monetary value in any way.
Laser Marking of Indicia on the Surface of Gemstones
Among the various techniques that have been developed for the permanent marking of gemstones, laser marking has been known for a long time in the gemstone industry. A preferred method for laser marking relies on the use of a laser beam with suitable characteristics, the beam being directed on a polished surface portion of a gemstone. Some key characteristics of the beam such as the average power or energy per pulse, the focusing conditions, the wavelength and the duration of the laser exposure are chosen so as to ablate a shallow layer of the surface material. Various types of laser systems have been proposed and used for laser marking of gemstones. For example, U.S. Pat. Nos. 5,149,938, 5,410,125, and 5,573,684 all to Winston et al., U.S. Pat. No. 6,187,213 to Smith, U.S. Pat. Nos. 6,483,073, 6,593,543, 6,747,242, and 6,788,714 all to Benderly disclose the use of excimer lasers capable of delivering ultraviolet laser radiation, i.e., laser radiation having a wavelength shorter than about 400 nm (nm: nanometre, 1 nm=10−9 m). Laser beams of shorter wavelength are preferred because the diameter of the engraved spots and the width of the engraved line segments scale with the wavelength of the beam. Note that most natural diamonds are of type Ia. Their ultraviolet absorption edge occurs at a wavelength of about 291 nm, so that they are substantially transparent for wavelengths in the visible region, which spans from 400 nm to about 700 nm. Nevertheless, solid-state laser systems have also been found attractive as laser sources for marking gemstones, particularly when their primary output beam is frequency doubled to get a final output wavelength typically in the range of 500 nm to 600 nm in the visible region. The use of Nd:YAG laser sources for engraving at the surface of gemstones has been disclosed in U.S. Pat. No. 4,392,476 to Gresser et al., U.S. Pat. No. 4,467,172 to Ehrenwald et al., U.S. Pat. No. 5,753,887 to Rosenwasser et al., and U.S. Pat. No. 6,713,715 to Christensen et al., while the use of Nd:YLF lasers has been taught in U.S. Pat. Nos. 5,932,119, 6,211,484, 6,476,351, and 6,684,663 all to Kaplan et al. Laser beams having a sizeable cross-sectional area when hitting the surface of the workpiece can produce ablated patterns with complex shapes through the use of a mask in which are machined cut-outs that precisely reproduce the shape of the desired pattern. Alternatively, indicia having complex patterns can be etched with a laser beam tightly focused to a very small spot at the surface of the workpiece. In this purpose, the workpiece can be mounted on a motorized XYZ translation stage with pre-programmed displacements. Another approach consists in using a beam steering apparatus to scan in a controlled manner the laser beam over a limited surface area of a workpiece, which is held immobile. Even with a tight focusing, the average power or energy per pulse available from a laser source can be insufficient to reach the surface ablation threshold of precious gemstones such as diamonds, which are made up of a very hard and generally transparent material. In this case, a light-absorbing material such as a dye or ink coating can be deposited on the surface of the workpiece prior to exposure to the laser beam. An alternative to the deposition of light-absorbing coatings is the use of a pulsed laser source capable of emitting laser pulses of duration less than about 1 ns (ns: nanosecond, 1 ns=10−9 s) to lower the threshold energy for vaporization of most materials, as taught in U.S. Pat. No. 6,713,715 to Christensen et al.
The indicia engraved using variations of the general technique as disclosed in the patents cited above do not impair either the appearance nor the grading of gemstones because the marks are generally engraved on a surface portion of the girdle of the gemstones. In particular, the marks engraved on diamonds often show some darkening due to the growth of a superficial layer of graphite during the laser ablation process. In many circumstances, the presence of graphite is of minor concern and, in fact, it may help to provide a better visibility of the indicia when they are intended to be read using a low magnification loupe. If desired, the layer of graphite can be removed with a surface treatment. An example of such a treatment is recited in U.S. Pat. No. 4,467,172 to Ehrenwald et al, and it consists in the application of 700° C. of heat combined to hydrochloric acid. Besides the highly contrasting appearance of the indicia caused by the presence of a layer of graphite in the etched surface areas, any indicium can be made more easy to detect and to recognize simply by enlarging it. An advantage of inscribing easily visible indicia having sizeable dimensions is that they may act as efficient theft deterrents in some particular situations.
Unfortunately, visible indicia inscribed directly on the surface of gemstones can be easily counterfeit by a simple repolishing of the engraved surface portion of the girdle or by using other types of surface treatments, this operation being possibly followed by the marking of a new but illicit indicium. A surface treatment aimed at defeating an indicium engraved on the surface of a gemstone would consist for example in removing any trace of graphite in the etched pattern, if any, and then to fill in the etched regions with a kind of fracture-filling product well known in the art. Even though the marking on a surface portion of the girdle does not detract from the appearance and grading of a gemstone, an indicium inscribed on the girdle may become hidden if the marking is carried out on a loose gemstone, prior to mounting it in a setting. Many settings have grips that prevent from getting visual access to the entire surface of the girdle.
In some other circumstances, however, it can be desired that the identification marking be as covert as possible to prevent unauthorized detection. An obvious way to reach this goal is to inscribe indicia of very small overall dimensions. As mentioned previously, the size of the smallest features that can be inscribed with a laser beam focused with conventional optics is fundamentally limited by the wavelength of the light, reaching what it is called the optical diffraction limit. Unfortunately, powerful, reliable and affordable laser sources emitting at wavelengths shorter than about 190 nm and configured for use in industrial environments are still lacking.
A major advance in the existing methods for laser marking at the surface of gemstones has been realized by using a special technique known as near-field optics. U.S. Pat. No. 6,624,385, U.S. application Ser. No. 10/607,184 and U.S. application Ser. No. 10/607,185 all to Patton et al. disclose the use of near-field optics for the marking of gemstones with a variety of laser sources such as excimer lasers and frequency-doubled Nd:YAG lasers. This technique enables the inscription of “micro-indicia” made up of features having dimensions well below what it is allowed by the optical diffraction limit. Near-field optics can be implemented by delivering the laser light through tapered optical fibers or, more preferably, through the use of a solid immersion lens whose flat output surface is set in close contact with a surface portion of a gemstone.
In addition to the known shortcomings of the laser marking on the surface of gemstones, marking micro-indicia of very small dimensions can make them difficult to locate in a reasonable time delay. Generally, a search key must be provided or the micro-indicia must be inscribed at precise locations relative to some obvious landmarks on the stone, such as the geometric centre of the table. In addition, the reading of subtle micro-indicia is generally performed through the use of complex and expensive devices. Finally, a counterfeiter can easily repolish the whole outer surface of a stolen gemstone to eliminate traces of any imperceptible micro-indicium.
Laser Marking of Indicia in the Volume of Transparent Materials
Independently of its overall size and complexity, an indicium can be made very difficult if impossible to counterfeit by engraving it well below the surface of a gemstone while leaving the exterior surface unaltered by the marking process. The layer of material located between the indicium and the exterior surface then acts as a thick protective barrier, so that altering the indicium becomes very difficult without inflicting severe and irreversible damages to the article marked in this way. Methods for sub-surface marking with a laser beam have been developed to mark objects whose properties, dimensions, and uses differ radically from those of common gemstones. For example, U.S. Pat. No. 5,206,496 to Clement et al. discloses the sub-surface laser marking of areas of increased opacity in the body of transparent materials such as glasses and plastics. The technique has been proposed for the marking of containers that serve for example to contain expensive fragrances that are distributed to a limited number of authorized retail outlets. Marking in the volume of a material offers the advantage of not only being able to withstand any surface treatment (including repolishing) aimed at destroying the indicium, but also of being very difficult to replicate by counterfeiters. Laser marking below the surface of diamonds is briefly taught in U.S. Pat. No. 4,467,172 to Ehrenwald et al, but no details are provided about the control of the shape, dimensions, and depth of the sub-surface occluded marks.
The inscription of marks (also referred to as “microstructures”) in the bulk of various transparent materials with a laser beam is a concept that offers great promise for the writing of two- and even three-dimensional arrays of densely packed point-like marks for permanent optical data storage applications. The concept is also attractive for building optical waveguides that serve to channel light in the bulk of optical materials such as fused silica. Both types of applications mentioned above call for the use of a write laser beam with tightly controlled temporal and spatial characteristics in order to inscribe microstructures of precise dimensions and shapes in the volume of a transparent material without inflicting any undesired optical damage to the bulk of the material or to its outer surface. While being primarily focused on optical information storage applications, U.S. Pat. No. 5,761,111 to Glezer discloses the use of ultrashort laser pulses to produce crack-free, regularly-shaped microstructures of high-contrast refractive index in a variety of transparent materials. These materials include fused silica, plastics, semiconductors, sapphire, and even fine crystals and jewelry. Three different marking regimes are discussed in the above-cited patent, the first one providing better control of the shape and dimensions of the inscribed microstructures. This regime relies on the use of a tightly focused pulsed laser beam with extremely short pulse duration, i.e., in the range of a few fs (fs: femtosecond, 1 fs=10−15 s) to about 200 ps (ps: picosecond; 1 ps=10−12 s). Another requirement of this specific marking regime relates to the energy carried by each laser pulse, which must be comparable or a few times higher than the threshold energy required to induce permanent structural changes (damages) in the host transparent material, for the selected laser wavelength and focusing characteristics.
Successful demonstration results of this sub-surface marking technique have been reported in the above-cited patent and in journal papers such as E. N. Glezer et al, “Three-dimensional optical storage inside transparent materials”, Optics Letters, Vol. 21, pp. 2023-2025, (1996), and E. N. Glezer et al., “Ultrafast-laser driven micro-explosions in transparent materials”, Applied Physics Letters, Vol. 71, pp. 882-884, (1997). For example, the authors succeeded in writing a two-dimensional array of low-contrast refractive index microstructures spaced from each other by about 2 μm (μm: micrometre, 1 μm=10−6 m) and having diameters in the range of 200-250 nm when observed from the face on which the write laser beam was incident. The microstructures were wntten at a depth of 100 μm below the surface of a recording medium made of fused silica. However, the patent and the related journal papers cited above failed to report on any successful attempt at marking in the bulk of a diamond material. In fact, the above references merely mention that the energy threshold for inducing structural changes in the bulk of diamonds is higher than those of most other transparent materials by a factor of at least 100.
Laser Marking in the Volume of Diamonds
Intrigued by the inconclusive situation just described above, and presumably unaware of U.S. Pat. No. 4,467,172 to Ehrenwald et al., J. B. Ashcom undertook more systematic experimental studies aimed at marking in the bulk of natural Ia and IIa single-crystal diamond samples with femtosecond laser pulses. He reported on his main results in Chapt. 4 of the Ph.D. thesis entitled “The Role of Focusing in the Interaction of Femtosecond Laser Pulses with Transparent Materials” (Harvard University, Cambridge, Mass., January 2003). Ashcom observed that directing a train of femtosecond laser pulses on the same spot in a diamond sample may produce optical damage (microstructures) in the bulk of the sample, but only when focusing the laser pulses with a microscope objective having a numerical aperture in the range of about 0.25 to 0.45. Ashcom undoubtedly succeeded in marking microstructures at a depth of about 40 μm below the surface of a diamond sample, using laser pulses carrying an energy that was varied in the range from about 20 nJ (nanoJoules) to 90 nJ. Surprisingly, a salient feature of his experimental investigations is the observation that even at the highest energy level and for the greatest number of pulses he used, there were instances where no internal damage was produced in the natural diamond samples. Likewise, there was a significant statistical component to the onset of the laser-induced damage from site to site in the same diamond sample, as well as from sample to sample. Spatial variations in the concentration of impurities present in its natural diamond samples were postulated as the cause of such a stochastic behaviour. The Senior thesis of an another member of the same group (J. C. Hwang, Harvard University, Cambridge, Mass., April 2003) also reports that the created microstructures had a dark and opaque appearance, which was hypothetically attributed to the presence of graphite, and more likely to the formation of amorphous carbon inside of each microstructure. Being aware of such results, Ashcom concluded that the successful marking in the bulk of diamonds was unlikely.
The crucial role played by the impurities and defects in the creation of marks in the bulk of a gemstone material is more clearly evidenced from the photomicrograph shown in FIG. 1A. Five laser pulses of about 150-fs duration and carrying an energy per pulse of about 500 nJ were focused all in the same volume within a natural diamond sample. Instead of a single mark centered on the peak of the focused beam intensity profile, FIG. 1A shows that at least three distinct marks have been created, each of them being located out of the volume in which the write laser beam got its narrowest transverse spot size. The local optical fluence at the position of each dark spot visible in the figure was then significantly lower than the peak fluence of the write laser beam, but it was nevertheless sufficient to initiate structural changes at places where well-localized defects and impurities were present in the material. FIG. 1B presents a further evidence of the localized nature and random distribution of the naturally-occurring defects and impurities. The figure shows a photomicrograph taken over a surface area of a natural diamond sample over which a tightly-focused femtosecond laser beam was translated along a linear trajectory at a constant velocity of 1 mm/s. The laser pulses of 50 μJ of energy were delivered at a rate of 1 kHz, and the trace shown in the figure spreads over a length of about 2 mm. The photomicrograph shows that the trace inscribed in the bulk of this specific natural diamond sample is far from being continuous, since it is made up of small dark spots randomly distributed along the trajectory. A striking feature of the photomicrograph is the presence of a long segment of the trace, located in the center region of the figure, that is free from any dark spot. On the other hand, the dark spots appear densely packed in some regions of the left-hand portion of the trace. In addition, many of these spots are located either above or below the center line of the trajectory, meaning that they have been formed in sites where the local optical fluence of the beam was not at its maximum peak level.
From the results presented in FIGS. 1A and 1B, it can be concluded that an appropriate choice of the energy per pulse is important to the successful marking of microstructures in natural diamond samples. For instance, if the energy per pulse is excessive, as it was the case in the example shown in FIG. 1A, several off-centered marks can be formed around (and slightly above) the targeted volume in the material. On the other hand, shooting with laser pulses having insufficient energy can result in failure to mark in volumes where defects are presumably absent. It is then expected that the proper range of energy per pulse may vary from site to site in the same natural diamond sample to get rid of the localized nature and random distribution of the defects from which the creation of the microstructures is initiated. The energy per pulse also impacts heavily on the subsequent growth of the inscribed marks. For example, FIG. 5C shows a photomicrograph taken across a surface area of a natural diamond sample in which a set of marks have been inscribed with a train of five laser pulses. The energy per pulse was in the range of a few μJ, and it was varied from site to site. The marks visible in FIG. 5C as black areas with irregular contours were inscribed in a natural diamond sample that was previously cut to give it the shape of a cube. The cubic shape allows the visual observation of the marks from any flat side wall of the sample, thus giving precious information about the spread of the microstructures along a direction parallel to the propagation axis of the write laser beam. In FIG. 5C, the write laser beam was then incident on a surface of the sample located at the top of the figure, and it propagated parallel to the downward direction in the figure. In this specific example, the extent of the microstructures along the vertical direction reaches more than 100 μm at the highest energy level used in the tests, as shown for both marks located in the right-most portion of the figure. As a result, both marks appear as dark spots with a diameter of about 30 μm when observed from the surface of incidence of the sample.
It was found that once a structural change has been initiated from a defect or impurity in a diamond material, the subsequent growth of the mark can be controlled by a proper selection of the key parameters of the marking process, such as the energy per pulse, the number of laser pulses directed onto each site within the sample, and the focusing characteristics of the write laser beam. However, a combination of laser parameters that is found suitable for a specific site in a gemstone material does not necessarily hold for any other site in the same gemstone, thus preventing from the development of a universal laser marking protocol. In fact, any operative laser marking protocol must include a real-time monitoring of the growth of each individual mark in order to stop the laser marking once the mark has the desired overall dimensions. This aspect is important for the inscription of indicia that do not detract from the appearance and grading of the marked gemstones.
In view of the prior art recited above and of the various problems and challenges reported when implementing the related techniques for laser inscription of indicia on the surface or below the surface of gemstones, there is a need for a method and a system that would enable reliable, safe, and controlled marking of indicia in the bulk of gemstones such as diamonds. There is also a need for a system that can account for the stochastic nature and variations in the marking processes developed so far, along with the peculiar physical properties of the natural diamonds in the formation of laser-induced microstructures therein.